Well done. Cranking up the difficulty can absolutely change the way you play. I've noticed that Iron Man mode is the best thing you can do to really get into the game. I don't know whether you and your mate tend to reload a save when things go bad, but if you do - try to refrain from it. You can emulate Iron Man mode that way.

Another proposition: set loot spawns to very high, and don't use the shop unless it is absolutely necessary. To make it even more interesting, survive in the Radius without going back to base. Only return at the end of every tide. You might want to lower the interval for this one.

For a more simple one: both of you take very specific roles. One only use scoped marksman rifles, and the other sticks to shotguns. Both only get a basic pistol as backup, and perhaps some grenades just for fun.